1. Field of the Invention
The present invention relates generally to gas turbine engine axisymmetric vectorable nozzles and, more particularly, to a positioning apparatus for the nozzle's divergent flaps and seals.
2. Discussion of the Background Art
One type of conventional gas turbine engine exhaust nozzle includes primary and secondary exhaust flaps arranged for defining a variable area converging-diverging exhaust nozzle. The exhaust nozzle is axisymmetric, or annular, and exhaust flow is confined by the primary and secondary flaps being positioned circumferentially adjacent to each other, respectively.
The secondary flaps, for example, have a forward end defining a throat of minimum flow area and an aft end having a larger flow area for defining a diverging nozzle extending downstream from the throat. The secondary flaps are variable, which means that the spacing between the secondary flaps as they are moved from a smaller radius position to a larger radius position must necessarily increase. Accordingly, conventional exhaust nozzle flap seals are suitably secured between adjacent ones of the secondary flaps to confine the exhaust flow and prevent leakage of exhaust flow between the secondary flaps.
An advanced axisymmetric vectoring nozzle (AVEN.TM. nozzle) has been developed and patented in U.S. Pat. No. 4,994,660, entitled "AXISYMMETRICAL VECTORING EXHAUST NOZZLE" by Hauer, assigned to the present assignee, and herein incorporated by reference. An axisymmetric vectoring nozzle provides a means for vectoring the thrust of an axisymmetric convergent/divergent nozzle by universally pivoting the divergent or secondary flaps of the nozzle in an asymmetric fashion or in other words pivoting the secondary flaps in radial and tangential directions with respect to the unvectored nozzle centerline.
Vectoring nozzles, and in particular axisymmetric vectoring exhaust nozzles of the type disclosed in the Hauer reference, provide positionable secondary flaps. These secondary flaps are positionable not only symmetrically, relative to a longitudinal centerline of the exhaust nozzle, but may also are positionable asymmetrically relative thereto for obtaining pitch and yaw vectoring. In such a vectored position, adjacent secondary flaps are positioned askew from each other, and therefore the exhaust nozzle flap seal disposed therebetween must be effective for maintaining sealing also as the secondary flaps are positioned for vectoring.
In the current AVEN.TM. nozzle the mechanical vector angle is generally limited by the seals bumping on the compression side of the divergent nozzle, where the flaps are angled away from the nozzle's centerline, or the divergent flap and seal unfeathering on the suction side of the divergent nozzle, where the flaps are angled away from the nozzle's centerline. There is a significant amount of vectoring capability which cannot be used because the mechanical limits due to bumping and/or unfeathering are encountered before the AVEN.TM. nozzle's exit area control actuator stroke limits are reached. The prior art means to avoid damaging the divergent nozzle and the AVEN.TM. nozzle's mechanical hardware is by limiting the amount of vectoring in the control software.
An alternative means to increase the vectoring angle without exceeding the bump and unfeather limits is to increase exit area to throat area ratio of the nozzle, referred to as A9/A8, where the exit area is as A9 and the throat area is as A8. In general, for a given throat area A8 a maximum vector angle is attainable at a particular A9/A8 which may or may not be optimal at that particular flight condition. This method may be used for small throat areas A8 at which optimal conditions may be available. Otherwise, this increase in A9/A8 area ratio reduces nozzle efficiency, consequently reducing gross thrust, and making the exhaust plume more susceptible to separation from the suction side of the vectored divergent flaps. Again in order to avoid plume separation during vectoring, the prior art nozzle system design uses control logic to attenuate the vector angle as it approaches empirically predetermined plume separation limits. Both of the vectoring angle limiting designs reduces the potential maneuverability and/or performance capability of the engine and aircraft.
There exists a need for a means to increase the vectoring angle limits of an AVEN.TM. nozzle beyond the prior art mechanical bump and/or unfeather limitations to the fullest extent possible for a given actuator stroke.